A) Field of the Invention
The present invention relates to a semiconductor device manufacture method, and more particularly a method for manufacturing a semiconductor device having a gate insulating film which contains nitrogen.
B) Description of the Related Art
In order to improve the integration degree and operation speed of a semiconductor integrated circuit device, MOSFETs as its constituent elements are made micro fine and gate insulating films are made thinner. A gate electrode on the gate insulating film is usually made of a polysilicon layer or a lamination of a polysilicon layer and a silicide layer. Impurity ions are usually implanted into the polysilicon layer at the same time when source/drain regions are formed. The gate electrode and source/drain regions of a surface channel type n-channel MOSFET are implanted with n-type impurities. The gate electrode and source/drain regions of a surface channel type p-channel MOSFET are implanted with p-type impurities.
As the gate insulating film becomes thin, there occurs the phenomenon that boron as p-type impurities implanted into the gate electrode of a surface channel type p-channel MOSFET pierces through the gate insulating film and reaches the channel region. As boron is implanted into the n-type channel region, not only the threshold value is changed but also the mobility is lowered.
It is known that introducing nitrogen into the gate insulating film is effective for suppressing piercing of boron through the gate insulating film. As a method of introducing nitrogen into a silicon oxide film, it is known to heat a silicon substrate by resistor heating or lamp heating in a nitridizing gas atmosphere such as NH3 gas, NO gas and N2O gas. Another method is also known which introduces nitrogen at a higher concentration in the surface layer of a silicon oxide film by using nitrogen plasma.
Another phenomenon is also known. As a gate insulating film becomes thin, tunneling current flows between the gate electrode and channel region, increasing gate leak current. If a gate insulating film of silicon oxide is (partially) replaced with an insulating film having a higher dielectric constant, a physical film thickness can be made thicker and gate leak current can be suppressed while an inversion capacitance equivalent film thickness is suppressed thin. Silicon oxynitride has generally a higher dielectric constant than that of silicon oxide so that it is effective for thickening a physical film thickness while the inversion capacitance equivalent film thickness is suppressed thin.
Japanese Patent Laid-open Publication No. 2002-198531 proposes to introduce nitrogen into a gate insulating film of silicon oxide formed on a silicon substrate by a remote plasma nitridizing process, and the gate insulation film is oxidized and nitridized in an N2O atmosphere at 800° C. to 1100° C. to redistribute nitrogen and form the gate insulating film having a uniform nitrogen concentration. This Publication describes that a transistor of long life and high reliability can be formed by forming a gate insulating film having a uniform nitrogen concentration at 6 at % or higher, e.g., 8 at % or 10 at %.
The remote plasma nitridizing process is a nitridizing process in which nitrogen plasma is generated by microwaves or the like in a plasma generation chamber independently from a processing chamber in which a substrate is accommodated and into which active plasma is introduced.
It can be considered that annealing in an N2O atmosphere may dissolve a portion of N2O gas into N2, O2, NO and the like, posing the problem that an oxide film thickness increase amount and a nitrogen concentration increase amount are hard to be controlled uniformly in each wafer and in wafers.
Japanese Patent Laid-open Publication No. 2002-110674 proposes to introduce nitrogen more on a gate insulating film surface in order to reduce gate leak current, while a nitrogen concentration near at an Si substrate interface is suppressed low, because the mobility of a MOS transistor is lowered if nitrogen is introduced into a region near at the Si substrate interface. This Publication proposes to perform radical nitridation using nitrogen gas for a silicon oxynitride film introduced with nitrogen beforehand, to suppress a nitrogen flow to be diffused from the surface, to suppress an introduced amount of nitrogen into a region near at the Si substrate interface, and to raise a nitrogen concentration in the film surface layer.